Brake system for a motor vehicle

ABSTRACT

A brake system for a motor vehicle includes a master brake cylinder in which a primary piston for generating a brake pressure in a hydraulic brake circuit can be displaced according to actuation of a brake pedal. It is possible to displace the primary piston under the effect of a hydraulic servo pressure circuit in the master brake cylinder. The hydraulic servo pressure circuit includes a pressure source for generating the servo pressure and a pressure accumulator for maintaining a minimum servo pressure in the servo pressure circuit, wherein the discharge line is constructed with a non-return valve which only allows discharge of hydraulic fluid from the pressure accumulator into the servo pressure circuit. A pressure sensor for detecting the hydraulic pressure prevailing in the pressure accumulator is associated with the pressure accumulator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional patent application of U.S. patentapplication Ser. No. 11/255,621, filed Oct. 21, 2005, which was acontinuation of International Application No. PCT/EP2004/003652 filedApr. 6, 2004 which claimed priority to German Patent Application No. 10318 850.9 filed Apr. 25, 2003, the disclosures of all three applicationsare incorporated herein by reference.

BACKGROUND

Various embodiments of a brake system for a motor vehicle are describedherein. In particular, the embodiments described herein relate to animproved brake system for a motor vehicle. In one embodiment, the brakesystem for a motor vehicle comprises a master brake cylinder in which aprimary piston for generating a brake pressure in a hydraulic brakecircuit can be displaced according to actuation of a brake pedal, itbeing possible to displace the primary piston under the effect of ahydraulic servo pressure circuit in the master brake cylinder and thehydraulic servo pressure circuit comprising a pressure source forgenerating the servo pressure and a pressure accumulator for maintaininga minimum servo pressure.

Brake systems of this type are known from the prior art. Thus DE 195 42656 A1, and corresponding U.S. Pat. No. 6,290,307 B1, both of which areincorporated by reference herein, for example discloses a brake systemfor a motor vehicle in which actuation of a brake pedal is detected bysensors. A servo pressure circuit is controlled by a control unit withthe aid of the detected pedal actuation. A pressure source is providedin this servo pressure circuit and generates a servo pressure in thehydraulic servo pressure circuit. The pressurised hydraulic fluid issupplied by means of a manifold valve to the master brake cylinder. Moreprecisely, the primary piston that can be displaced inside the masterbrake cylinder by means of the servo pressure for generating a brakepressure is also pressurised with a servo force, resulting from theservo pressure, in addition to the pedal actuation force. As a result,the primary piston can be effectively displaced inside the master brakecylinder without the entire force required for generating the brakeforce having to be applied by the brake pedal.

In order to be able to provide a sufficiently high servo pressure at anyinstant during operation, the servo pressure circuit of DE 195 42 656 A1provides a pressure accumulator connected downstream of the pressuresource. This pressure accumulator is directly coupled to the servopressure circuit. Whenever the pressure source of the servo pressurecircuit is active hydraulic fluid is conveyed from the servo pressurecircuit into the pressure accumulator and stored therein. However, thishas the effect that in a state in which there is a relatively lowhydraulic pressure in the servo pressure circuit and in which the fullpower of the pressure source would be inherently required for generatinga brake pressure inside the master brake cylinder for displacing theprimary piston, a certain portion of the hydraulic fluid conveyed by thepressure source will also always be conveyed into the pressureaccumulator owing to the fluidic connection of the pressure accumulatorand pressure source. The efficiency and the response characteristic ofthe brake system suffers as a consequence, in particular as not all ofthe hydraulic fluid required for actuating the primary piston isconveyed from the servo pressure circuit into the master brake cylinder.

SUMMARY

The present application describes various embodiments of a brake systemfor a motor vehicle. In one embodiment, a brake system is provided forwhich at the outset of operation the servo pressure circuit operates ina manner suitable to the situation and with increased effectiveness.

Such increased effectiveness is achieved by a brake system for a motorvehicle comprising a master brake cylinder in which a primary piston forgenerating a brake pressure in a hydraulic brake circuit can bedisplaced according to actuation of a brake pedal, it being possible todisplace the primary piston under the effect of a hydraulic servopressure circuit in the master brake cylinder and the hydraulic servopressure circuit comprising a pressure source for generating the servopressure and a pressure accumulator for maintaining a minimum servopressure. The pressure accumulator of the servo pressure circuit can besupplied with hydraulic fluid as a function of the servo pressureprevailing in the servo pressure circuit.

In such a brake system, the pressure accumulator is supplied withhydraulic fluid contained in the servo pressure circuit or conveyed bythe pressure source only if current operation of the entire brake systemalso allows it. Such a mode of supply takes place, for example, if nobraking is currently being carried out which requires loading of theprimary piston inside the master brake cylinder with hydraulic fluidflowing in from the servo pressure circuit. In such a situation thepressure source can be used to “load” the pressure accumulator, in otherwords to produce a pressure level therein which can subsequently be usedin a later operating state to equalise pressure fluctuations. If,however, braking is initiated by actuation of the brake pedal, thepossibility of supplying the pressure accumulator with hydraulic fluidoriginating from the pressure source is limited or even prevented, sohydraulic fluid conveyed by the pressure source can be suppliedsubstantially completely to the master brake cylinder. As a result theprimary piston inside the master brake cylinder can be quickly andeffectively displaced for generating a brake pressure. The embodimentsdescribed and illustrated herein thus defy the drawbacks of the brakesystem according to DE 195 42 656 A1 discussed at the outset and allowsoperation of the various components of the servo pressure circuit in amanner suitable to the situation.

To allow selective supplying of the pressure accumulator as a functionof the current operating state of the brake system and still allowreliable discharge of hydraulic fluid from the pressure accumulator toequalise pressure fluctuations it is provided in one embodiment of thebrake system that the pressure accumulator is fluidically connected by asupply line and/or a discharge line to the servo pressure circuit. As aresult of the construction of the brake system with two at leastpartially separate lines for supplying the pressure accumulator withhydraulic fluid and for discharging hydraulic fluid from the pressureaccumulator it is possible to reliably achieve situation-dependentcontrol of the pressure accumulator. However, it is also possible toprovide only one line which can be used both for supplying the pressureaccumulator with hydraulic fluid and for discharging hydraulic fluidfrom the pressure accumulator.

The supply line comprises a fluid supply controller for controlling thesupply of hydraulic fluid to the pressure accumulator as a function ofthe servo pressure prevailing in the servo pressure circuit. A fluidsupply controller of this type can be constructed so as to be passivelyor actively controllable. Thus, it is possible that the fluid supplycontroller comprises a pressure-limiting valve. The pressure-limitingvalve can, for example, only open if a specific minimum pressure isachieved or exceeded. This means that the pressure source in the servopressure circuit has adjusted a sufficiently high hydraulic pressure bymeans of which reliable operation of the brake system, in particularreliable displacement of the primary piston inside the master brakecylinder, is possible in the event of actuation of the brake pedal. Ifthis state is attained, the pressure-limiting valve opens and thepressure accumulator can be filled, in other words “loaded,” with thehydraulic fluid conveyed by the pressure source. A correspondingpressure level is adjusted in the pressure accumulator in the process.

In another embodiment of the brake system, the fluid supply controllercomprises a throttle element. The use of a throttle element for thefluid supply controller in the supply line also allowsfunction-dependent control of the pressure accumulator. The throttleelement thus allows supplying of the pressure accumulator with hydraulicfluid conveyed by the pressure source only in the event of highhydraulic pressures. In other words, the throttle element, as a flowdivider, to a certain extent also means that the hydraulic fluidconveyed by the pressure source is supplied to the master brake cylinderwith higher priority and only on attaining a specific minimum pressureis a certain portion of the hydraulic fluid conveyed by the pressuresource also conveyed to the pressure accumulator.

The pressure-limiting valve and the throttle element may be connected inseries. A combination of this type of pressure-limiting valve andthrottle valve in the supply line allows the quantity of hydraulic fluidsupplied to the pressure accumulator and the pressure level that hasbuilt up therein to be limited as a result of the throttling effect ofthe throttle element, even in the event of the minimum pressure, presetby the pressure-limiting valve, for opening the pressure-limiting valveand for supplying the pressure accumulator being exceeded.

A pressure sensor or a plurality of pressure sensors may be provided atvarious points for monitoring the servo pressure prevailing in the servopressure circuit. In this connection, another embodiment of the brakesystem provides that the fluid supply controller comprises an activelycontrollable manifold valve, in particular a two-way solenoid valve. Theactively controllable manifold valve can be controlled with the aid ofthe data captured by the pressure sensor. For example this activelycontrollable manifold valve can, on reaching a specific minimumpressure, be transferred from its closed state into its open state andbe closed again after exceeding a specific minimum pressure in the servopressure circuit. It is also possible to construct the pressure-limitingvalve or the throttle element, both discussed above, so as to beactively controllable. Thus for example the pressure-limiting valve canalso be controllable in such a way that the minimum pressure thereofthat determines the operating state is changed as a function of theoperating situation of the brake system. When using an activelycontrollable throttle element the throttling setting thereof can bechanged as a function of the servo pressure currently prevailing in theservo pressure circuit.

In another embodiment, the discharge line is constructed with anon-return valve. To avoid undesirable hydraulic fluid flows via thedischarge line the non-return valve can be used between the pressureaccumulator and the hydraulic line of the servo pressure circuitconnecting the pressure source to the master brake cylinder in such away that it only allows discharge of hydraulic fluid from the pressureaccumulator into the servo pressure circuit. The non-return valve can,for example, also be replaced by an actively controllable component.

In another embodiment, the pressure source comprises a motor-drivenpump. It may also be provided that the servo pressure circuit comprisesa non-return valve connected downstream of the pressure source. Thenon-return valve is oriented such that it allows conveying of hydraulicfluid from the pressure source into the servo pressure circuit, inparticular in the direction of the master brake cylinder, whereas itprevents flowing back of hydraulic fluid to the pressure source.Consequently it can be ensured that the servo pressure that has built upin the servo pressure circuit does not reduce in the direction of thepressure source in the event of power fluctuations in the pressuresource. In particular, the non-return valve can prevent the servopressure emanating from the pressure accumulator and exerted on theservo pressure circuit from discharging in the direction of the pressuresource but in the direction of the master brake cylinder.

As described above, the servo pressure circuit is used to displace theprimary piston inside the master brake cylinder to generate a brakepressure in the brake circuit. For the event that the servo pressurecircuit fails, for example because the pressure source fails, it should,however, be ensured that the primary piston can also be displaced insome other way, for example by a direct mechanical coupling to the brakepedal. In such a case the situations where the actuating force acting onthe primary piston simultaneously also leads to recirculation ofhydraulic fluid inside the servo pressure circuit or actuation of theprimary piston is obstructed owing to the occurrence ofabove-atmospheric pressure or a vacuum in the servo pressure circuitshould be avoided. For this reason, and for general shielding of themaster brake cylinder and servo pressure circuit, another embodiment ofthe brake system provides that the servo pressure circuit comprises anactively controllable manifold valve, in particular a two-way solenoidvalve, interconnected between the master brake cylinder and the pressuresource and the pressure accumulator. The servo pressure circuit can bedecoupled from the master brake cylinder by means of the activelycontrollable manifold valve and undesirable interactions can be ruledout. This actively controllable manifold valve can also be generallyused for controlling the servo pressure acting on the primary piston. Itmay also be used to actively fill the pressure accumulator. Thus it ispossible, for example, to close the actively controllable manifold valveafter a braking operation has ended and to continue to operate the pumpover a predetermined period. As a result, above-atmospheric pressure isgenerated in the portion of the servo pressure circuit that is close tothe pump and shut by the actively controllable manifold valve, it beingpossible to use the pressure to supply the pressure accumulator withhydraulic fluid. If at the end of braking the servo pressure in theservo pressure circuit was already sufficiently high to supply thepressure accumulator, the pump can also be switched off as early as atthe end of braking and the servo pressure prevailing in the servopressure circuit can initially be used for supplying the pressureaccumulator before this is reduced in some other way, for example in thedirection of a hydraulic fluid reservoir.

The servo pressure circuit may comprise a further actively controllablemanifold valve, in particular a two-way solenoid valve, interconnectedbetween the master brake cylinder and the hydraulic fluid reservoir. Asa result it is possible to connect the master brake cylinder to thehydraulic fluid reservoir and thus to reduce servo pressure in the servopressure circuit after a braking operation has ended. A further manifoldvalve of this type may also ensure that no above-atmospheric pressure orvacuum builds up in the servo pressure circuit and obstructs or evenblocks the primary piston displacement in the event of a mechanicalactuation of the primary piston caused by an emergency.

The case was discussed above where pressure is no longer properlygenerated in the servo pressure circuit, for example owing to a failureof the pressure source. It is equally possible for the servo pressurecircuit to supply excessive servo pressure which could possibly lead toan overreaction of the brake system. To prevent a scenario of this typea development, one embodiment of the brake system provides that theservo pressure circuit comprises a bypass channel that can be activatedas a function of pressure and is connected downstream of the pressuresource. The bypass channel can be opened or closed as a function of theprevailing servo pressure and thus hydraulic fluid can be removed fromthe servo pressure circuit in the event of excessive servo pressure.During normal operation, i.e. if the servo pressure inside the servopressure circuit is below a predetermined maximum value, the bypasschannel is blocked.

In another embodiment of the brake system, a pressure sensor fordetecting the hydraulic pressure prevailing in the pressure accumulatoris associated with the pressure accumulator. As already generally statedabove with respect to the servo pressure circuit, a separate pressuresensor can, in particular, also be associated with the pressureaccumulator. The instantaneous “loading state” of the pressureaccumulator can therefore be ascertained and evaluated with the aid ofthe pressure prevailing in the accumulator, for example to the extentthat in the event of a specific minimum pressure not being attainedinside the pressure accumulator and with the pressure source powersimultaneously being available an active fluid supply control element isopened and hydraulic fluid is supplied to the pressure accumulator forincreasing the accumulator pressure.

Reference should be made to the fact that a low-pressure accumulator ispreferably used as the pressure accumulator, the maximum accumulatorpressure of which is limited. While a low-pressure accumulator of thistype is suitable only for temporarily bridging a pressure drop, forexample owing to starting delays in the motor drive of the pressuresource, it does have the advantages that it is inexpensively available,requires only a small installation space and is usually sufficientduring operation. The main load of the pressure generation generallyfalls to the pressure source anyway. With respect to the use of alow-pressure accumulator, a pressure sensor associated therewithsatisfies the further task of also detecting attainment of a maximumpressure, whereupon an additional hydraulic fluid is to be supplied tothe pressure accumulator in order to prevent damage or destruction tothe low-pressure accumulator. Obviously the maximum pressure value to bedetected by the pressure sensor should be selected with sufficientsecurity with respect to a pressure value at which damage can actuallyoccur to the pressure accumulator.

As already illustrated above, the brake system may be constructed with alarge number of active components and pressure sensors. In thisconnection, one embodiment of the brake system provides a controller forevaluating detected hydraulic pressures and for evaluating activelycontrollable components. The controller can, for example, be formed byan electronic data processing system present in a motor vehicle anyway.It can, however, also be formed by a separate data processing systemwith a separate processor.

In the above description, it was generally illustrated that the brakesystem operates according to actuation of the brake pedal. This can meanthat, on the one hand, the actuating force exerted on a brake pedal ismechanically transmitted to the primary piston and that actuation of theprimary piston is assisted by the servo pressure circuit. In other wordsthe pedal actuating force can be detected and accordingly intensified bythe servo pressure circuit. As an alterative to this it is alsopossible, during normal operation, to completely mechanically decouplethe primary piston and its movement from brake pedal actuation. Thismeans that pedal actuation is accordingly no longer transmitted directlyto the primary piston, rather the energy causing it dissipates. Theprimary piston is displaced in such a configuration of the brake systemexclusively during normal operation, under the effect of the servopressure circuit. As a result, during normal operation the servopressure circuit alone is responsible for displacement of the primarypiston. For emergency operation, in which, for example, a component ofthe servo pressure circuit fails, a mechanical coupling of brake pedaland primary piston may be provided and be produced, for example, afterbridging a movement pedal.

In another embodiment of the brake system for a motor vehicle, the brakesystem comprises a master brake cylinder, in particular of theabove-described type, in which, according to actuation of a brake pedal,but mechanically decoupled therefrom, a primary piston can be displacedfor generating a brake pressure in a hydraulic brake circuit, it beingpossible to displace the primary piston under the effect of a hydraulicservo pressure circuit in the master brake cylinder and the hydraulicservo pressure circuit comprising a pressure source for generating theservo pressure. The servo pressure circuit may comprise a bypass channelthat can be activated as a function of pressure and is connectedupstream of the master brake cylinder.

Other advantages of the brake system for a motor vehicle will becomeapparent to those skilled in the art from the following detaileddescription, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic general diagram of a first embodiment of abrake system;

FIG. 2 shows a schematic general diagram according to FIG. 1 of a secondembodiment of a brake system;

FIG. 3 shows a schematic general diagram according to FIGS. 1 and 2 of athird embodiment of a brake system;

FIG. 4 shows a schematic general diagram according to FIGS. 1 to 3 of afourth embodiment of a brake system; and

FIG. 5 shows a schematic general diagram according to FIGS. 1 to 4 of afifth embodiment of a brake system.

DETAILED DESCRIPTION

FIG. 1 shows a first embodiment of a brake system which is designatedgenerally at 10. The brake system comprises a servo pressure circuit 12and a brake cylinder subassembly 14.

The brake cylinder subassembly 14 comprises a master brake cylinder 16in which a primary piston 18 is displaceably guided. A secondary piston20 is also displaceably guided in the master brake cylinder 16 andmechanically coupled to the primary piston 18 by a spring arrangement.The primary piston 18, with the master brake cylinder 16 and thesecondary piston 20, encloses a primary pressure chamber 22. Thesecondary piston 20, with the master brake cylinder 16, encloses asecondary pressure chamber 24. The primary pressure chamber 22 and thesecondary pressure chamber 24 are fluidically coupled by respectivesupply channels to a hydraulic fluid accumulator 26 for supplyinghydraulic fluid in the rest state shown in FIG. 1. The primary pressurechamber 22 and the secondary pressure chamber 24 are also fluidicallycoupled to a brake system 28 which, in a manner known per se, can bringabout deceleration of the wheels of a motor vehicle.

The brake cylinder subassembly 14 also comprises a pedal simulationdevice 30. In detail, the pedal simulation device 30 comprises a forceinput member 32 mechanically coupled to a brake pedal and which isworkingly connected to a working piston 34. The working piston 34 can bedisplaced in a piston chamber 36, it limiting working chambers on eitherside inside the piston chamber 36. A gas contained in the workingchambers on either side of the working piston 34 is displaced from oneworking chamber through a throttle 38 (as shown in FIG. 1) into therespective other working chamber by a displacement of the working piston34 inside the piston chamber 36 and, as a result, due to the effect ofthe throttle 38 a resistance to a movement of the force input member 32is generated. A resistance which results from a simulation springarrangement 40 acts on the force input member 32 in addition to thepneumatically generated resistance. This simulation spring arrangement40 is constructed in multiple stages, i.e. it comprises a spring withlow spring hardness and a spring with increased spring hardness whichcan be reduced by steps, i.e. compressed with a progressive springcharacteristic.

Any pedal actuation, i.e. any displacement of the force input member 32,is detected by a position sensor 42, the position sensor 42 emitting aposition signal to an electronic control unit 44, with the aid of whichthe existence of a pedal actuation can be ascertained.

The brake system 10 according to FIG. 1 is designed in such a way that apedal actuation, and a displacement of the force input member 32resulting therefrom, is not mechanically transmitted to the primarypiston 18, rather the energy applied during the pedal actuation isdissipated in the brake system 10. To carry out braking, hydraulicpressure is generated in an actuating pressure chamber 46 by means ofthe servo pressure circuit 12, which pressure displaces the primarypiston 18, and consequently the secondary piston 20, and as a resultprovides for a pressure build up in the primary pressure chamber 22 andin the secondary pressure chamber 24. The primary piston 18 is thuscompletely mechanically decoupled from the force input member 32 duringnormal operation of the brake system 10.

The construction of the servo pressure circuit 12 is to be describedhereinafter. The servo pressure circuit 12 is supplied with hydraulicfluid from the hydraulic fluid reservoir 26. This is conveyed from thehydraulic fluid reservoir 26 via a supply line 48 by a pump 50 which isdriven by a motor 52. The pump 50 is fluidically coupled by a hydraulicfluid supply line 54 to the actuating pressure chamber 46. A branch 56,which is connected in parallel with the pump 50 and which comprises apressure-limiting valve 58, is provided in the hydraulic fluid supplyline 54 so as to issue from the pump 50. A non-return valve 60 isaccordingly integrated into the hydraulic fluid supply line 54 andallows fluid to flow from the pump 50 into the actuating pressurechamber 46, but blocks a flow of fluid in the opposite direction. Adischarge line 62 comprising a non-return valve 64 subsequently opensinto the hydraulic fluid supply line 54. The non-return valve 64 isoriented in such a way that it blocks a flow of fluid from the hydraulicfluid supply line 54 into the discharge line 62 but allows fluid to flowin the opposite direction. A pressure accumulator 66 is coupled to thedischarge line 62. The pressure accumulator 66 is constructed as alow-pressure accumulator and is configured in such a way that hydraulicfluid can be stored therein up to a specific maximum pressure. Thepressure accumulator is constructed in an inherently conventional mannerand is coupled at its side remote from the non-return valve 64 to abackflow line 68. The pressure accumulator 66 also comprises a springelement 70 which is used for pressure accumulation.

A supply line 72 subsequently branches from the hydraulic fluid supplyline and is likewise provided with a pressure-limiting valve 74. Thesupply line 72 is hydraulically coupled to the pressure accumulator 66.

Starting from the branching of the supply line 72, a two-way valve 76that can be actively controlled by the electronic control unit 44 isarranged in the further course of the hydraulic fluid supply line 54.This, as shown in FIG. 1, can be switched into a closed state and intoan open state. Finally, a further bypass channel 78, which likewisecomprises a two-way valve 80 that can be controlled by the electroniccontrol unit 44, adjoins the further course of the hydraulic fluidsupply line 54. A further pressure sensor 82, which detects the servopressure currently prevailing in the hydraulic fluid line 54 andforwards it to the electronic control unit 44 in the form of acorresponding signal, is arranged in the further course of and on thehydraulic fluid supply line 54.

The servo pressure circuit 12 functions as follows. The motor 52, whichmotively drives the pump 50, is controlled according to the control ofthe electronic control unit 44. The pump conveys hydraulic fluid fromthe hydraulic fluid reservoir 26 into the hydraulic fluid supply line54. The actuating pressure chamber 46 is supplied with hydraulic fluidby the hydraulic fluid supply line 54, so a pressure builds up in thischamber, if required, which pressure, in the event of actuation of thebrake pedal and displacement of the force input member 32, leads,without mechanical coupling of force input member 32 and primary piston18, to the primary piston 18 being displaced to the left in FIG. 1 underthe effect of the hydraulic pressure that has built up in the actuatingchamber 46. As a result, a hydraulic pressure builds up in the primarypressure chamber 22 and, with displacement of the secondary piston 20,in the secondary pressure chamber 24.

With sufficiently high servo pressure in the hydraulic fluid supply line54, the pressure-limiting valve 74 opens in the supply line 72, sohydraulic fluid can flow into the pressure accumulator 66 via the supplyline 72. If, however, the servo pressure prevailing in the hydraulicfluid supply line 54 exceeds a maximum value, the pressure-limitingvalve 58 opens and short-circuits the entire portion of the servopressure circuit 12 adjoining the branch 56. As a result, overburdeningof the servo pressure circuit 12 may be prevented. If, on the otherhand, the servo pressure prevailing in the hydraulic fluid supply line54 sinks below a specific minimum value, and if the pressure accumulator66 is sufficiently “loaded,” the pressure accumulator 66 can, ifrequired, temporarily equalise a drop in pressure. Such a drop inpressure can, for example, come about because an actuation of the brakepedal (not shown) switches the two-way valve 46 over from its closedposition shown in FIG. 1 into an open state and servo pressure is thussupplied from the servo pressure circuit 12 to the actuating pressurechamber 56. However, owing to a starting delay of the motor 52 the pump50 cannot build up servo pressure in the servo pressure circuit 12quickly enough, so a pressure drop briefly occurs. This is equalised bythe pressure accumulator 66. The two-way valve 76, controlled by theelectronic control unit 44 according to the signal obtained by theposition sensor 42 and further signals, for example from the pressuresensor 82, thus acts as an activating element for activation of theservo pressure circuit 12. The two-way valve 80 also acts as a pressurerelief valve for the hydraulic fluid supply line 54. It is switched intoits open position shown in FIG. 1, for example, if there is no brakeactuation and thus a build up of pressure is not necessary in theactuating pressure chamber 46 either. As a result, it may also allowpressure equalisation between the master brake cylinder and thereservoir 26. This is significant in particular with respect to apossible defect in the servo pressure circuit 12 during which theprimary piston 32 can be mechanically moved. Opening the two-way valve80 prevents such a mechanical movement of the primary piston 32 frombeing obstructed by above-atmospheric pressure or a vacuum occurring inthe servo pressure circuit 12 as a result. As soon as, during normaloperation with a completely functional servo pressure circuit 12, abrake actuation is detected by the position sensor 42 the two-way valve80 is switched into its closed position so hydraulic fluid can besupplied by the hydraulic fluid line 54 to the actuating pressurechamber 46.

A fundamental aspect of the embodiments of the brake system describedherein lies in the fluidic connection of the pressure accumulator 66 inthe servo pressure circuit 12. In the solution shown in FIG. 1, it ispossible for the pressure accumulator 66 to be supplied with hydraulicfluid if there is a sufficiently high servo pressure in the servopressure circuit 12, i.e. if the pump 50 conveys enough hydraulic fluidfrom the hydraulic fluid accumulator 26. The pressure accumulator 66, ifit is appropriately loaded, can equalise pressure fluctuations insidethe servo pressure circuit 12. This is necessary for example if pressurecannot be built up sufficiently quickly in the servo pressure circuit 12owing to abrupt braking and owing to a starting delay in the unitcomprising pump 50 and motor drive 52.

For supplying the pressure accumulator there is also the possibilitythat the two-way valve 76 is closed after a braking operation has endedand the pump 50 continues to be operated over a predetermined period. Asa result hydraulic pressure, which can be used to load the pressureaccumulator 66, builds up in the portion of the hydraulic fluid supplyline 54 between the pump 50 and the two-way valve 76. In the event thatthe servo pressure is already sufficiently high once the brakingoperation has ended, in order to load the pressure accumulator 66 thepump 50 can also be directly switched off once the braking operation hasended and when the two-way valve 80 is closed and the two-way valve 76is open the pressure accumulator 66 can be loaded.

FIG. 2 shows a second embodiment of the brake system. To provide asimpler description and to avoid repetitions only the differences fromthe first embodiment according to FIG. 1 shall be described. The samereference numerals as in the description of the first embodimentaccording to FIG. 1 will be used for identical or identically-actingcomponents, but will be preceded by the number “1.”

The only difference between the embodiment according to FIG. 1 and thesecond embodiment according to FIG. 2 lies in the fact that, in thesupply line 172, the pressure-limiting valve 74 provided in FIG. 1 wasbeen replaced in FIG. 2 by a throttle element 186. The throttle element186 fulfills the function of limiting a supply of hydraulic fluid viathe supply line 172 to the pressure accumulator 166 as a function ofpressure. The throttle element 186 is used as a volume flow divider, thelarger portion of the volume flow conveyed by the pump 150 flowingthrough the hydraulic fluid supply line 154 from the pump 150 into theactuating pressure chamber 146 and only a small portion is supplied viathe throttle element 186 and the supply line 172 to the pressureaccumulator 166. However, in the event of high servo pressure inside thehydraulic fluid supply line 154 a sufficiently high quantity ofhydraulic fluid is supplied via the throttle element 186 and the supplyline 172 to the pressure accumulator 166 and the latter loaded thereby.A gas contained in the working chambers on either side of the workingpiston 134 is displaced from one working chamber through a throttle 138(as shown in FIG. 2) into the respective other working chamber by adisplacement of the working piston 134 inside the piston chamber 136.

The remaining mode of operation of the brake system 110 corresponds tothe mode of operation of the brake system 10 from FIG. 1 and illustratedin detail.

FIG. 3 shows a third embodiment of a brake system. Again only thedifferences from the embodiments according to FIG. 1 and 2 will bedescribed to avoid repetitions. For this purpose, use will again be madeof the reference numerals from the preceding description of theembodiments according to FIG. 1 and 2 for identically-acting andidentical components, the reference numerals being preceded by thenumber “2.”

The embodiment according to FIG. 3 shows a combination of the twoembodiments according to FIG. 1 and 2. Firstly the pressure-limitingvalve 274 and following it the throttle element 286 are connected inseries into the supply line 272. As a result it is possible to throttlethe hydraulic flow via the throttle element 286 even in the event ofsufficiently high servo pressure inside the servo pressure circuit 212at which the pressure-limiting valve 274 opens for supplying thepressure accumulator 266. As a result the quantity of hydraulic fluidsupplied to the pressure accumulator 266 and the hydraulic pressure thathas built up therein can be reduced, even in the event of sufficientlyhigh hydraulic pressure, as the throttle element 286 again acts as avolume flow divider. With this solution it is possible, in any pressureregion, as long as it lies below the maximum pressure value determiningthe function of the pressure-limiting valve 258, to moderately supplythe pressure accumulator 266. A construction of this type is recommendedin particular when using a low-pressure accumulator 266 which is tobe-protected from being supplied with excessive hydraulic pressure. Agas contained in the working chambers on either side of the workingpiston 234 is displaced from one working chamber through a throttle 238(as shown in FIG. 3) into the respective other working chamber by adisplacement of the working piston 234 inside the piston chamber 236.

Apart from that the brake system 210 according to FIG. 3 functions asdescribed above with reference to FIG. 1 with respect to the firstembodiment.

FIG. 4 shows a fourth embodiment of the brake system. Again only thedifferences from the preceding embodiments according to FIG. 1 to 3 areto be described. The same reference numerals, as were used in thedescription of FIG. 1 to 3, will again be used for this description foridentical or identically-acting components but will be preceded by thenumber “3.”

In the fourth embodiment according to FIG. 4, a two-way valve 388, whichcan be controlled by the electronic control unit 344, is fitted into thesupply line 372 leading from the hydraulic fluid supply line 354 to thepressure accumulator 366. The supply line 372 also comprises a pressuresensor 390 between the pressure accumulator 366 and the two-way valve388. The hydraulic pressure prevailing in the pressure accumulator 366can be detected by means of the pressure sensor 390. If this hydraulicpressure falls below a predetermined minimum value and the pump 350supplies a sufficiently high servo pressure in the servo pressurecircuit 312 the two-way valve 388 is switched from its closed positionshown in FIG. 4 into the open position. The hydraulic fluid can thusflow unhindered from the hydraulic fluid supply line 354 into thepressure accumulator 366 and thus load the pressure accumulator 366. Thechange in the pressure level in the pressure accumulator 366 during thisloading process is monitored by the pressure sensor 390 and theelectronic control unit 344. As soon as a desired level or a maximumpermitted pressure level is attained in the pressure accumulator 366 thepressure sensor 390 detects this and emits a corresponding signal to theelectronic control unit 344. The unit then controls the two-way valve388 so the latter returns to its closed position shown in FIG. 4 andthus the fluidic connection via the supply line 372 to the hydraulicfluid supply line 354 is broken. The pressure accumulator 366 may thusbe purposefully loaded as a function of pressure by active switching ofthe two-way valve 388 with the embodiment according to FIG. 4. A gascontained in the working chambers on either side of the working piston334 is displaced from one working chamber through a throttle 338 (asshown in FIG. 4) into the respective other working chamber by adisplacement of the working piston 334 inside the piston chamber 336.

Apart from that the brake system according to FIG. 4 functions asdescribed above with respect to FIG. 1 to 3 with regard to embodiments 1to 3.

It should also be noted that, owing to the effect of the two-way valve388, the discharge line 362 can be omitted in a development (not shown)of the embodiment according to FIG. 4. The pressure accumulator 366 issupplied and emptied via the line 372 in this case.

FIG. 5 shows a fifth embodiment of the brake system. Again only thedifferences from the above-described embodiments according to FIG. 1 to4 are to be described. The same reference numerals as above for thedescription of FIG. 1 to 4 will be used for identically-acting andidentical components, but they will be preceded by the number “4.”

In the fifth embodiment according to FIG. 5, the servo pressure circuit412 is illustrated in a highly simplified manner compared with theabove-described embodiments. The circuit includes only a pressure sourcecomprising pump 450 and motor 452 and which can be short-circuited bythe branch 456 and the pressure-limiting valve 458 located therein. Thenon-return valve 460 is also provided as is the bypass channel 478 withthe two-way valve 480 integrated therein and controllable by theelectronic control unit 444.

In this embodiment of the brake system, the pump 450 and the motor 452are constructed in such a way that on activation by a brake pedalactuation, they can react sufficiently quickly without starting delayand thus a servo pressure can build up sufficiently quickly inside theservo pressure circuit 412, which pressure then provides for adisplacement of the primary piston 418 in the actuating pressure chamber446. Such a fast-reacting construction of the pressure source,comprising pump 450 and motor drive 452 allows a much simplifiedconstruction of the servo pressure circuit. A gas contained in theworking chambers on either side of the working piston 434 is displacedfrom one working chamber through a throttle 438 (as shown in FIG. 5)into the respective other working chamber by a displacement of theworking piston 434 inside the piston chamber 436.

The embodiment of a brake system described herein show a simple andreliable possibility for providing a master brake cylinder with a servopressure according to a brake pedal actuation.

Reference should be made to the fact that the embodiments of a brakesystem described herein have been described in connection with a brakesystem in which, during normal operation, complete mechanical decouplingis provided between primary piston and force input member. It is,however, equally possible to use the brake systems described herein inwhich there is no such mechanical decoupling between force input member(and thus the brake pedal) and primary piston and the servo pressureonly acts in a supportive manner to displace the primary piston directlymoved by actuation of the brake pedal.

In accordance with the provisions of the patent statutes, the principleand mode of operation of the brake system for a motor vehicle have beenexplained and illustrated in its various embodiments. However, it mustbe understood that the brake system for a motor vehicle described hereinmay be practiced otherwise than as specifically explained andillustrated without departing from its spirit or scope.

1. Brake system for a motor vehicle comprising a master brake cylinderin which a primary piston for generating a brake pressure in a hydraulicbrake circuit can be displaced according to actuation of a brake pedal,it being possible to displace the primary piston under the effect of ahydraulic servo pressure circuit in the master brake cylinder and thehydraulic servo pressure circuit comprising a pressure source forgenerating the servo pressure and a pressure accumulator for maintaininga minimum servo pressure in the servo pressure circuit, wherein thedischarge line is constructed with a non-return valve which only allowsdischarge of hydraulic fluid from the pressure accumulator into theservo pressure circuit, and wherein a pressure sensor for detecting thehydraulic pressure prevailing in the pressure accumulator is associatedwith the pressure accumulator.
 2. Brake system according to claim 1,wherein the servo pressure circuit comprises a manifold valve, inparticular a two-way solenoid valve, that is actively controllablyinterconnected between the master brake cylinder and the pressure sourceand the pressure accumulator.
 3. Brake system according to claim 2,wherein the servo pressure circuit comprises a further manifold valve,in particular a two-way solenoid valve, that is actively controllablyinterconnected between the master brake cylinder and a hydraulic fluidreservoir.
 4. Brake system according to claim 1, wherein the servopressure circuit comprises a bypass channel that can be activated as afunction of pressure and is connected downstream of the pressure source.5. Brake system according to claim 1, further comprising a controllerfor evaluating detected hydraulic pressures and for controlling activelycontrollable components.
 6. Brake system according to claim 1, whereinthe primary piston is mechanically decoupled from the brake pedal. 7.Brake system for a motor vehicle comprising a master brake cylinder,wherein a primary piston for generating a brake pressure in a hydraulicbrake circuit can be displaced according to actuation of a brake pedal,it being possible to displace the primary piston under the effect of ahydraulic servo pressure circuit in the master brake cylinder, and thehydraulic servo pressure circuit comprising a pressure source forgenerating the servo pressure, and a pressure accumulator formaintaining a minimum servo pressure in the servo pressure circuit, thepressure accumulator being fluidically connected by a supply line to theservo pressure circuit, and the supply line comprising a fluid supplycontroller for controlling the supply of hydraulic fluid to the pressureaccumulator as a function of the servo pressure prevailing in the servopressure circuit, characterised in that the fluid supply controllercomprises an actively controllable manifold valve, in particular atwo-way solenoid valve.
 8. Brake system according to claim 7, wherein apressure sensor is arranged in the servo pressure circuit for detectingthe servo pressure.
 9. Brake system according to claim 7, wherein themanifold valve is controllable according to the servo pressure detectedby the pressure sensor.
 10. Brake system according to claim 7, whereinthe pressure source comprises a motor-driven pump.
 11. Brake systemaccording to claim 7, wherein the servo pressure circuit comprises anon-return valve connected downstream of the pressure source.
 12. Brakesystem according to claim 7, wherein the servo pressure circuitcomprises a manifold valve, in particular a two-way solenoid valve, thatis actively controllably interconnected between the master brakecylinder and the pressure source and the pressure accumulator.
 13. Brakesystem according to claim 12, wherein the servo pressure circuitcomprises a further manifold valve, in particular a two-way solenoidvalve, that is actively controllably interconnected between the masterbrake cylinder and a hydraulic fluid reservoir.
 14. Brake systemaccording to claim 7, wherein the servo pressure circuit comprises abypass channel that can be activated as a function of pressure and isconnected downstream of the pressure source.
 15. Brake system accordingto claim 7, wherein a pressure sensor for detecting the hydraulicpressure prevailing in the pressure accumulator is associated with thepressure accumulator.
 16. Brake system according to claim 7, furthercomprising a controller for evaluating detected hydraulic pressures andfor controlling actively controllable components.
 17. Brake systemaccording to claim 7, wherein the primary piston is mechanicallydecoupled from the brake pedal.